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Status and Conservation Efforts of Ashy Storm on the Farallon Islands.

Abstract

The Ashy Storm-Petrel (Oceanodroma homochroa) is one of the least known seabird species because of its crevice-nesting and nocturnal habits. The Farallon National Wildlife Refuge supports the largest population of Ashy Storm-Petrels. Results from a capture-recapture study indicate that the population of Ashy storm-petrels on the Farallon Islands has declined almost 40% in 20 years and predicts that the population will continue to decline at a rate of almost 3% per year. Reasons for this decline include predation by gulls, mice, and owls. Current conservation efforts include studies to better document predation on Ashys, efforts to exclude gulls from Ashy nesting habitat, and a combination of artificial nesting boxes and playback systems to promote the recovery of this seabird species.

Introduction

The Ashy storm-petrel (Oceanodroma homochroa) is a small seabird, weighing approximately 42 grams and just over seven inches in length. This species belongs to the order Procellariiformes, the same group as albatrosses, shearwaters, and fulmars. Storm-petrels are the smallest members of this group, and because they are mostly nocturnal in their breeding activities, are amongst some of the least known seabird species. Ashy storm-petrels nest in rock walls and crevices and have a limited breeding range, primarily on islands off the coast of central and southern California, and in smaller numbers on Islas Los Coronados, Mexico. The Farallon Islands, located 27 miles west of San Francisco, hosts the most significant proportion of this seabird population. Ashy storm-petrels have an extended breeding season, lasting six months or more, during which time a single chick is raised.

The Farallon National Wildlife Refuge supports the largest breeding population for the Ashy Storm-Petrel and its native predator, the Western Gull. Ten additional seabird species breed on the Farallon Islands, totaling approximately 200,000 nesting seabirds. Such a large assemblage is remarkable for this small group of islands, which together comprise approximately 120 acres. Since its declaration as a National Wildlife Refuge in 1909, the U.S. Fish and Wildlife Service have managed the Farallon Islands.

Population in decline

It is believed that the Southeast Farallon Islands supports between 50 - 70% of the world population of Ashys. Under a cooperative agreement with USFWS since 1969, Point Reyes Bird Observatory (PRBO) has been conducting long-term monitoring of this population. Results from capture-recapture studies indicates that the population has declined almost 40% in 20 years from an estimated 3,500 - 4,000 breeding birds in 1972 to approximately 2,000 - 2,400 birds in 1992. Furthermore, demographic modeling predicts that the Ashy population will continue to decline at a rate of almost 3% per year. Currently, there is no official listing for the Ashy stormpetrel under the Endangered Species Act, although it has been designated a species of special concern by the State of California and species of management concern by the U.S. Fish and Wildlife Service.

Threats to reproduction

Several possible explanations exist for the population decline of Ashy storm-petrels on the Farallon Islands. One of the largest densities of Ashy storm-petrels on the Farallon Islands is on the south-facing slope of the hill where the lighthouse is located. Until recently, Western Gulls did not nest in these areas. In the 1980s however, gulls expanded their breeding range to include this prime petrel habitat. As a result, petrel remains are commonly encountered on the slopes of this hill. Although Western Gulls are native on the island, their population has been increasing as a result of the activities of man. In an attempt to discourage gulls from nesting in areas important for Ashy storm-petrels, USFWS constructed gull exclosures consisting of a system of cables strung between poles. Other avian predators of Ashys include Burrowing Owls, which are resident on the island in the fall, winter, and spring.

Another possible threat to the breeding Ashy population is the presence of the introduced House mouse (Mus musculus), which is capable of entering crevices and taking eggs and even small chicks. On other islands, introduced animals have had devastating effects on seabird populations. Currently, it is unknown what impact mice have on the Ashy population, although plans to document mouse and gull predation on Ashys are underway.

Conservation efforts

Artificial nesting boxes have been successfully used as tools for aiding in the restoration of seabird populations on the Farallones and elsewhere for seabirds such as Cassin's Auklets, Rhinoceros Auklets, Tufted Puffins, and Pigeon Guillemots. Social attraction is based on vocalization playback of the species that is being attracted and has also been a successful tool for bringing seabirds to nesting areas. To increase good "Ashy habitat" on the Farallones and to promote the recovery of this species, a combination of nesting boxes and vocalization playback has been implemented. Future plans include continuing to develop effective methods of decreasing gull densities in areas of important Ashy habitat, as well as better documentation of Ashy storm-petrel predation and predators, in an attempt to preserve this unique and beautiful seabird species.

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Kyra L. Mills Point Reyes Bird Observatory, Stinson Beach, CA 94970; kyramills@prbo.org
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Article Details
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Author:Mills, Kyra L.
Publication:Endangered Species Update
Date:Sep 1, 2000
Words:839
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